AAC VBR

Discuss Max, an open source CD audio extractor and audio converter.
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Mike1
Posts: 201
Joined: Thu Sep 21, 2006 3:55 pm

AAC VBR

Post by Mike1 » Sun Apr 01, 2007 10:55 pm

I just looked at iTunes 7.1.1 and there's a checkbox for VBR but that's it.

With Max, of course, VBR is further qualified by Minimum, Low, Medium, High, Maximum.

Don't they both use Core Audio? Why the difference?

Also, I haven't used AAC encoding much for a long time. But I just made a few files, transcoding from FLACs.

What I did was to look at the bitrates that LAME had used for the same tracks and moved the nominal bitrate up or down for the AAC encoder in accordance with that. So if LAME had given the tracks on a particular album (perhaps an old mono one) around, say, 100kbps, I set the nominal bitrate to 128; if it had given them anything between about 160 and 180, I set it to 160; and if it had used much over 190, I went for 192.

What I found was curious. I found that if the setting was 128 or 160, then the average bitrate of the resulting file would usually be around that. However, when the nominal bitrate was set to 192 (because LAME had given a particular track, say 191 or 196) then the bitrate would go through the roof. I was getting bitrates of 240 or more.

Does anyone know: is this a quirk of the encoder? I've trouble believing that the tracks in question were just outstandingly demanding.

I guess I'd sooner use the MP4 standard than the MP3 one, but I'm really not sure I trust Apple's encoder. I suppose this is quibbling, as any modern encoder sounds OK at a reasonable bitrate, but I'd like to be sure I was getting a good quality/filesize trade-off. It would be nice if the Core Audio AAC encoder was a true VBR one, where you could simply select a desired "quality" from 0 to 10 (or 0 to 1) as with Vorbis or Nero's AAC encoder. Maybe it will be with Leopard ...


EDIT: scratch that. I've tried a few more examples and it's not something that invariably happens.

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